Method of producing wear-resistant iron-based sintered alloy

ABSTRACT

A wear-resistant iron-based sintered alloy made of a mixed powder including first hard particles, second hard particles, graphite particles, and iron particles is produced. The first hard particles are Fe—Mo—Ni—Co—Mn—Si—C alloy particles. The second hard particles are Fe—Mo—Si alloy particles. The mixed powder includes the first hard particles at 5 mass % to 50 mass %, the second hard particles at 1 mass % to 8 mass %, and the graphite particles at 0.5 mass % to 1.5 mass % when a total amount of the above particles is set as 100 mass %. In a sintering process, sintering is performed so that the hardness of the first hard particles becomes 400 to 600 Hv and the hardness of the second hard particles exceeds 600 Hv. Then, an oxidation treatment is performed so that a density difference between before and after the oxidation treatment in a sintered product becomes 0.05 g/cm3 or more.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-074255 filed on Apr. 4, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a method of producing a wear-resistant iron-based sintered alloy including hard particles suitable for improving wear resistance of a sintered alloy.

2. Description of Related Art

A sintered alloy based on iron may be applied to a valve seat and the like. Hard particles may be included in the sintered alloy in order to further improve wear resistance. When hard particles are included, graphite particles and iron particles are mixed into hard particles to form a powder, and the mixed powder is compact-molded into a molded product for a sintered alloy. Then, generally, the molded product for a sintered alloy is heated and thus it is sintered and becomes a sintered alloy.

As a method of producing such a sintered alloy, a method of producing a wear-resistant iron-based sintered alloy in which a mixed powder in which hard particles, graphite particles, and iron particles are mixed is compact-molded into a molded product for a sintered alloy, and the molded product for a sintered alloy is sintered while C of the graphite particles of the molded product for a sintered alloy diffuses into the hard particles and the iron particles has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-156101 (JP 2004-156101 A).

Here, the hard particles include Mo: 20 mass % to 70 mass %, C: 0.2 mass % to 3 mass %, and Mn: 1 mass % to 15 mass %, with the balance including inevitable impurities and Co. The mixed powder includes the hard particles at 10 mass % to 60 mass % and the graphite particles at 0.2 mass % to 2 mass % when the total amount of the hard particles, the graphite particles, and the iron particles is set as 100 mass %. Since hard particles are dispersed into such a sintered alloy, it is possible to prevent abrasive wear.

SUMMARY

However, a matrix material connecting hard particles of a wear-resistant iron-based sintered alloy produced in the production method described in JP 2004-156101 A is soft because it is an Fe—C material in which C of the graphite particles has diffused into the iron particles. Therefore, when the wear-resistant iron-based sintered alloy and a metallic material of a sliding counterpart component that comes in contact therewith are in metal-contact with each other, a contact surface of the wear-resistant iron-based sintered alloy is likely to be plastically deformed and adhesive wear easily occurs on the contact surface. In order to prevent such problems, it is desirable to increase the hardness of the wear-resistant iron-based sintered alloy. However, there is a risk of the machinability of the wear-resistant iron-based sintered alloy deteriorating accordingly and it is difficult to achieve both adhesive wear resistance and machinability.

The present disclosure provides a method of producing a wear-resistant iron-based sintered alloy through which it is possible to secure machinability while preventing adhesive wear.

The inventors expected that the adhesive wear of a contact surface would accelerate when the iron matrix of a wear-resistant iron-based sintered alloy was plastically deformed as described above. In this regard, the inventors studied addition of other hard particles through which it is possible to prevent plastic deformation of an iron matrix in addition to the hard particles through which abrasive wear has so far been prevented. Therefore, the inventors focused on molybdenum as a main component of the hard particles, and found that, when an iron-molybdenum intermetallic compound and a molybdenum carbide precipitated during sintering are interspersed in an iron matrix, it is possible to control plastic deformation of the iron matrix. In addition thereto, the inventors acquired the new finding that, when oxidizing a part of the iron of the iron matrix derived from the iron particles to triiron tetraoxide, it is possible to improve the wear resistance thereof without deteriorating the machinability of the sintered alloy.

An aspect of the present disclosure relates to a method of producing a wear-resistant iron-based sintered alloy including a molding process in which a mixed powder including hard particles, graphite particles, and iron particles is compact-molded into a molded product for a sintered alloy; and a sintering process in which the molded product for the sintered alloy is sintered while C of the graphite particles of the molded product for the sintered alloy diffuses into the hard particles and the iron particles, wherein the hard particles include first hard particles and second hard particles, wherein the first hard particles include Mo: 20 mass % to 70 mass %, Ni: 5 mass % to 40 mass %, Co: 5 mass % to 40 mass %, Mn: 1 mass % to 20 mass %, Si: 0.5 mass % to 4.0 mass %, and C: 0.5 mass % to 3.0 mass %, with the balance including Fe and inevitable impurities when an amount of the first hard particles is set as 100 mass %, wherein the second hard particles include Mo: 60 mass % to 70 mass %, and Si: 2.0 mass % or less, with the balance including Fe and inevitable impurities when an amount of the second hard particles is set as 100 mass %, wherein the mixed powder includes the first hard particles at 5 mass % to 50 mass %, the second hard particles at 1 mass % to 5 mass %, and the graphite particles at 0.5 mass % to 1.5 mass % when a total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set as 100 mass %, and wherein, in the sintering process, sintering is performed so that the hardness of the first hard particles becomes 400 to 600 Hv and the hardness of the second hard particles exceeds 600 Hv, after the sintering process, an oxidation treatment is performed on a sintered product sintered from the molded product for the sintered alloy so that a part of iron contained in an iron matrix derived from the iron particles becomes triiron tetraoxide, and the oxidation treatment is performed so that a difference between a density of the sintered product before the oxidation treatment and a density of the sintered product after the oxidation treatment becomes 0.05 g/cm³ or more.

According to the present disclosure, it is possible to secure machinability while preventing adhesive wear.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic conceptual view of a wear test used in examples and comparative examples; FIG. 2 is a schematic conceptual view of a machinability test used in examples and comparative examples;

FIG. 3A is a graph showing results of wear test wear amount proportions with respect to amounts of first hard particles added in Examples 1 to 3 and Comparative Examples 1 and 9;

FIG. 3B is a graph showing results of tool wear amount proportions with respect to amounts of the first hard particles added in Examples 1 to 3 and Comparative Examples 1 and 9;

FIG. 4A is a graph showing results of wear test wear amount proportions with respect to amounts of second hard particles added in Examples 1, 4, and 5 and Comparative Examples 3, 4, and 9;

FIG. 4B is a graph showing results of tool wear amount proportions with respect to amounts of the second hard particles added in Examples 1, 4, and 5 and Comparative Examples 3, 4, and 9;

FIG. 5A is a graph showing results of wear test wear amount proportions with respect to amounts of graphite particles added in Examples 1, 6, and 7 and Comparative Examples 5, 6, and 9;

FIG. 5B is a graph showing results of tool wear amount proportions with respect to amounts of the graphite particles added in Examples 1, 6, and 7 and Comparative Examples 5, 6, and 9;

FIG. 6A is a graph showing results of wear test wear amount proportions with respect to the hardness of the first hard particles in Examples 1, 3, 5, and 8 and Comparative Examples 8 and 9;

FIG. 6B is a graph showing results of tool wear amount proportions with respect to the hardness of the first hard particles in Examples 1, 3, 5, and 8 and Comparative

Examples 8 and 9;

FIG. 7A is a graph showing results of wear test wear amount proportions with respect to the density difference in the sintered products in Examples 1 to 8 and Comparative Examples 7 and 9;

FIG. 7B is a graph showing results of tool wear amount proportions with respect to the density difference in the sintered products in Examples 1 to 8 and Comparative Examples 7 and 9;

FIG. 8A is a picture of a surface of a test piece according to Example 1 after a wear test;

FIG. 8B is a picture of a surface of a test piece according to Comparative Example 7 after a wear test;

FIG. 9A is a picture of a structure of the test piece according to Example 1;

FIG. 9B is a picture of a structure of a test piece according to Comparative Example 5;

FIG. 9C is a picture of a structure of a test piece according to Comparative Example 6;

FIG. 10A is a graph showing results of wear test wear amount proportions in Examples 1 and 9 and Comparative Example 10; and

FIG. 10B is a graph showing results of tool wear amount proportions in Examples 1 and 9 and Comparative Example 10.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present disclosure will be described below in detail. A molded product for a sintered alloy (hereinafter referred to as a molded product) according to the present embodiment is obtained by compact-molding a mixed powder including first and second hard particles, graphite particles, and iron particles to be described below. A wear-resistant iron-based sintered alloy (hereinafter referred to as a sintered alloy) is obtained by sintering the molded product while C of the graphite particles diffuses into the hard particles and the iron particles. The hard particles, the molded product obtained by compact-molding a mixed powder in which the hard particles are mixed, and the sintered alloy obtained by sintering the molded product will be described below.

1. First Hard Particles

The first hard particles are particles that are mixed as a raw material into the sintered alloy and have high hardness with respect to iron particles and an iron matrix of the sintered alloy, and thus prevent abrasive wear of the sintered alloy.

The first hard particles are particles made of a Co—Mo—Ni—Fe—Mn—Si—C alloy. Specifically, the first hard particles include Mo: 20 mass % to 70 mass %, Ni: 5 mass % to 40 mass %, Co: 5 mass % to 40 mass %, Mn: 1 mass % to 20 mass %, Si: 0.5 mass % to 4.0 mass %, and C: 0.5 mass % to 3.0 mass %, with the balance including Fe and inevitable impurities when the amount of the first hard particles is set as 100 mass %. In addition, Cr may be added to the first hard particles in a range of 10 mass % or less as necessary. The hardness of the first hard particles before sintering is preferably in a range of 400 to 600 Hv.

The first hard particles can be produced by preparing molten metal in which the above composition is mixed together in the above proportions and performing an atomization treatment in which the molten metal is atomized. In addition, as another method, a solidified body in which molten metal has solidified may be formed into a powder by mechanical grinding. As the atomization treatment, either a gas atomization treatment or a water atomization treatment may be performed. However, in consideration of sinterability and the like, a gas atomization treatment is more preferable because round particles are obtained.

Here, lower limit values and upper limit values of the above hard particle composition can be appropriately changed according to the degree of importance of characteristics of application components in consideration of reasons for limitation to be described below and the hardness, solid lubricity, adhesiveness, and cost in such ranges.

1-1. Mo: 20 Mass % to 70 Mass %

In the composition of the first hard particles, Mo can generate Mo carbide together with C of a carbon powder during sintering and improve the hardness and wear resistance of the first hard particles. In addition, regarding Mo, since under a high temperature usage environment, Mo in a solid solution state and Mo carbide are oxidized to form a Mo oxide film, it is possible to obtain favorable solid lubricity for a sintered alloy.

Here, when a content of Mo is less than 20 mass %, not only the amount of Mo carbide generated is reduced, but also an oxidation initiation temperature of the first hard particles increases, and generation of Mo oxide under a high temperature usage environment is prevented. Thereby, the solid lubricity of the obtained sintered alloy is insufficient, and the abrasive wear resistance thereof decreases. On the other hand, when a content of Mo exceeds 70 mass %, not only it is difficult to produce the first hard particles using an atomizing method, but also adhesiveness between the hard particles and the iron matrix decreases. More preferably, a content of Mo is 30 mass % to 50 mass %.

1-2. Ni: 5 Mass % to 40 Mass %

In the composition of the first hard particles, Ni can enlarge an austenitic structure of the matrix of the first hard particles and improve the toughness thereof. In addition, Ni can increase an amount of Mo in a solid solution state of the first hard particles, and improve the wear resistance of the first hard particles.

Further, Ni diffuses into the iron matrix of the sintered alloy during sintering, can enlarge the austenitic structure of the iron matrix, increase the toughness of the sintered alloy, increase an amount of Mo in a solid solution state in the iron matrix, and improve wear resistance.

Here, when a content of Ni is less than 5 mass %, the above effects of Ni are expected to be unlikely. On the other hand, when a content of Ni exceeds 40 mass %, although the above effects of Ni are maximized, the cost of the first hard particles increases. More preferably, a content of Ni is 20 mass % to 40 mass %.

1-3. Co: 5 Mass % to 40 Mass %

In the composition of the first hard particles, similarly to Ni, Co can enlarge the austenitic structure in the matrix of the first hard particles and the iron matrix of the sintered alloy, and improve the hardness of the first hard particles.

Here, when a content of Co is less than 5 mass %, the above effects of Ni are expected to be unlikely. On the other hand, when a content of Co exceeds 40 mass %, although the above effects of Co are maximized, the cost of the first hard particles increases. More preferably, a content of Co is 10 mass % to 30 mass %.

1-4. Mn: 1 Mass % to 20 Mass %

In the composition of the first hard particles, since Mn efficiently diffuses from the first hard particles into the iron matrix of the sintered alloy during sintering, it is possible to improve adhesiveness between the first hard particles and the iron matrix. In addition, Mn can enlarge the austenitic structure in the matrix of the first hard particles and the iron matrix of the sintered alloy.

Here, when a content of Mn is less than 1 mass %, since an amount of Mn diffusing into the iron matrix is small, the adhesiveness between the hard particles and the iron matrix decreases. Thus, the mechanical strength of the obtained sintered alloy decreases. On the other hand, when a content of Mn exceeds 20 mass %, the above effects of Mn are maximized. More preferably, a content of Mn is 2 mass % to 8 mass %.

1-5. Si: 0.5 mass % to 4.0 mass % In the composition of the first hard particles, Si can improve adhesiveness between the first hard particles and a Mo oxide film. Here, when a content of Si is less than 0.5 mass %, the above effects of Si are expected to be unlikely. On the other hand, when a content of Si exceeds 4.0 mass %, moldability of the molded product deteriorates and the density of the sintered alloy decreases. More preferably, a content of Si is 0.5 mass % to 2 mass %.

1-6. C: 0.5 Mass % to 3.0 Mass %

In the composition of the first hard particles, C combines with Mo to form Mo carbide, and can improve the hardness and wear resistance of the first hard particles. Here, when a content of C is less than 0.5 mass %, the wear resistance effect is not sufficient. On the other hand, when a content of C exceeds 3.0 mass %, moldability of the molded product deteriorates and the density of the sintered alloy decreases. More preferably, a content of C is 0.5 mass % to 2 mass %.

1-7. Cr: 10 Mass %

Hereinafter, in the composition of the first hard particles, Cr can prevent excessive oxidation of Mo during use. For example, addition of Cr is effective when a usage environment temperature of the sintered alloy is high, an amount of the Mo oxide film generated in the first hard particles increases, and the Mo oxide film peels off from the first hard particles,.

Here, when a content of Cr exceeds 10 mass %, formation of the Mo oxide film in the first hard particles is prevented too much. Here, under a corrosive environment such as an alcohol fuel environment, in order to improve corrosion resistance, it is desirable to add Cr. On the other hand, under an environment in which adhesive wear is likely to occur, in order to accelerate oxidation, it is desirable to reduce a content of Cr.

1-8. Particle Size of First Hard Particles

The particle size of the first hard particles can be appropriately selected according to an application, a type, and the like of the sintered alloy. However, the particle size of the first hard particles is preferably in a range of 44 μm to 250 μm, and more preferably in a range of 44 μm to 105 μm.

Here, when hard particles with a particle size less than 44 μm are included as the first hard particles, since the particle size is too small, wear resistance of the wear-resistant iron-based sintered alloy may decrease. On the other hand, when hard particles with a particle size greater than 250 μm are included as the first hard particles, since the particle size is too large, the machinability of the wear-resistant iron-based sintered alloy may deteriorate.

2. Second Hard Particles

Similarly to the first hard particles, the second hard particles are particles that are mixed as a raw material into the sintered alloy and have high hardness with respect to the iron particles and an iron matrix of the sintered alloy. The second hard particles are particles that significantly increase the hardness of the sintered alloy when added in a small amount, prevent plastic deformation of the iron matrix of the sintered alloy, and as a result, decrease adhesive wear of the sintered alloy.

The second hard particles are particles made of an Fe—Mo alloy, and include Mo: 60 mass % to 70 mass %, and Si: 2.0 mass % or less, with the balance including Fe and inevitable impurities when the amount of the second hard particles is set as 100 mass %. The hardness of the second hard particles before sintering is preferably in a range of 600 to 1600 Hv.

A solidified body in which molten metal has solidified is formed into a powder by mechanical grinding to produce the second hard particles. In addition, like the first hard particles, the second hard particles may be produced through a gas atomization treatment, a water atomization treatment, or the like.

2-1. Mo: 60 Mass % to 70 Mass %

In the composition of the second hard particles, Mo can generate Mo carbide together with C of a carbon powder during sintering and improve the hardness and wear resistance of the second hard particles. In addition, regarding Mo, since under a high temperature usage environment, Mo in a solid solution state and Mo carbide are oxidized to form a Mo oxide film, it is possible to obtain favorable solid lubricity for a sintered alloy. In addition, when molybdenum carbide precipitates at a grain boundary of the iron matrix during sintering, it is possible to prevent plastic deformation of the iron matrix and adhesive wear during use.

Here, when a content of Mo is less than 60 mass %, it is difficult to prevent plastic deformation of the iron matrix according to molybdenum carbide described above and adhesive wear resistance decreases. On the other hand, when a content of Mo exceeds 70 mass %, it is difficult to produce second hard particles using a grinding method, and a yield thereof decreases.

2-2. Si: 2.0 Mass % or Less

When Si is contained in the composition of the second hard particles, it is easy to produce the second hard particles using a grinding method. Here, when a content of Si exceeds 2.0 mass %, the hardness of the second hard particles increases, moldability of the molded product deteriorates, the density of the sintered alloy decreases, and also the machinability of the sintered alloy deteriorates.

2-3. Particle Size of Second Hard Particles

The particle size of the second hard particles can be appropriately selected according to an application, a type, and the like of the sintered alloy. However, the particle size (maximum particle size) of the second hard particles is preferably in a range of 100 μm or less and more preferably, 75 μm or less. Thereby, the second hard particles can be uniformly dispersed into the matrix and it is possible to increase the hardness of the sintered alloy. Here, when hard particles with a particle size greater than 100 μm are included as the second hard particles, since the particle size is too large, the machinability of the sintered alloy may deteriorate. Here, the particle size of the second hard particles is preferably 1μm or more in consideration of production.

3. Graphite Particles

The graphite particles may be either natural graphite particles or artificial graphite particles or a mixture thereof as long as C of the graphite particles can diffuse into the iron matrix and hard particles in a solid solution state during sintering. The particle size of the graphite particles is preferably in a range of 1μm to 45 μm. As a powder including preferable graphite particles, a graphite powder (CPB-S commercially available from Nippon Kokuen Group) can be exemplified.

4. Iron Particles

The iron particles serving as the matrix of the sintered alloy are iron particles containing Fe as a main component. As a powder including iron particles, a pure iron powder is preferable. However, a low alloy steel powder may be used as long as moldability during compact-molding does not deteriorate and diffusion of elements such as Mn of the above first hard particles does not decrease. As the low alloy steel powder, an Fe—C powder can be used. For example, a powder having a composition including C: 0.2 mass % to 5 mass %, the balance including inevitable impurities and Fe when the amount of the low alloy steel powder is set as 100 mass % can be used. In addition, such a powder may be a gas atomized powder, a water atomized powder or a reduced powder. The particle size of the iron particles is preferably in a range of 150 μm or less.

5. Mixing Ratio of Mixed Powder

A mixed powder including the first hard particles, the second hard particles, the graphite particles, and the iron particles is prepared. The mixed powder includes the first hard particles at 5 mass % to 50 mass %, the second hard particles at 1 mass % to 5 mass %, and the graphite particles at 0.5 mass % to 1.5 mass % when the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set as 100 mass %.

The mixed powder may include only the first hard particles, the second hard particles, the graphite particles, and the iron particles, but may include about several mass % of other particles as long as the mechanical strength and wear resistance of the obtained sintered alloy do not decrease. In this case, when the total amount of the first and second hard particles, the graphite particles, and the iron particles is 95 mass % or more with respect to the mixed powder, sufficient effects can be expected. For example, at least one type of particles for improving machinability selected from the group consisting of sulfides (for example, MnS), oxides (for example, CaCO₃), fluorides (for example, CaF), nitrides (for example, BN), and oxysulfides may be included in the mixed powder.

Since the first hard particles are included at 5 mass % to 50 mass % with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, it is possible to improve both the mechanical strength and the abrasive wear resistance of the sintered alloy.

Here, when the first hard particles are included at less than 5 mass % with respect to the total amount, as can be clearly understood from experiments performed by the inventors to be described below, a sufficient effect of the abrasive wear resistance according to the first hard particles cannot be exhibited.

On the other hand, when the amount of the first hard particles exceeds 50 mass % with respect to the total amount, since the amount of the first hard particles is too large, when the molded product is molded from the mixed powder, it is difficult to mold the molded product. In addition, since there is greater contact between the first hard particles and a part in which iron particles are sintered becomes smaller, the abrasive wear resistance of the sintered alloy decreases.

Since the second hard particles are included at 1 mass % to 5 mass % with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, as described above, it is possible to prevent plastic deformation of the iron matrix during use and reduce the adhesive wear of the sintered alloy.

Here, when a content of the second hard particles is less than 1 mass % with respect to the total amount, as can be clearly understood from experiments performed by the inventors to be described below, the adhesive wear resistance of the sintered alloy decreases. On the other hand, when a content of the second hard particles exceeds 5 mass % with respect to the total amount, the machinability of the sintered alloy deteriorates.

Since the graphite particles are included at 0.5 mass % to 1.5 mass % with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, after sintering, without melting the first and second hard particles, C of the graphite particles can diffuse in a solid solution state into the first and second hard particles, and additionally, a pearlite structure can be secured in the iron matrix. Therefore, it is possible to improve both the mechanical strength and the wear resistance of the sintered alloy.

Here, when a content of the graphite particles is less than 0.5 mass % with respect to the total amount, since the ferrite structure of the iron matrix is likely to increase, the strength of the iron matrix itself of the sintered alloy decreases. On the other hand, when a content of the graphite particles exceeds 1.5 mass % with respect to the total amount, a cementite structure precipitates and the machinability of the sintered alloy deteriorates.

6. Method of Producing Wear-Resistant Iron-Based Sintered Alloy

In this manner, the obtained mixed powder is compact-molded into a molded product for a sintered alloy (molding process). In the molded product for a sintered alloy, the first hard particles, the second hard particles, the graphite particles, and the iron particles are included at the same proportion as in the mixed powder.

While C of the graphite particles of the molded product for a sintered alloy diffuses into the first and second hard particles and the iron particles, the molded product for a sintered alloy that is compact-molded is sintered to produce a sintered product (sintering process). In this case, not only is there greater diffusion of iron from the iron matrix (iron particles) into the first and second hard particles but also the second hard particles do not contain carbon. Therefore, carbon of the graphite particles easily diffuses into the second hard particles, Mo carbide is generated at a grain boundary between the second hard particles, and the hardness of the sintered alloy can increase.

In the present embodiment, sintering is performed by adjusting a sintering temperature and a sintering time so that the hardness of the first hard particles becomes 400 to 600 Hv and the hardness of the second hard particles exceeds 600 Hv. Regarding the hardnesses of the first and second hard particles in the obtained sintered alloy, these hardnesses are values measured using a micro Vickers hardness testing machine at a measurement load of 0.1 kgf. When the hardness of the first hard particles is set to be within such a range, it is possible to secure wear resistance and machinability for the sintered alloy. Here, when the hardness of the first hard particles is less than 400 Hv, a difference in hardness from the iron matrix in which carbon is in a solid solution state is small, and the wear resistance of the sintered alloy decreases. On the other hand, when the hardness of the sintered alloy exceeds 600 Hv, the machinability of the sintered alloy may deteriorate.

In addition, when the hardness of the second hard particles is set to be within such a range, it is possible to improve the wear resistance of the soft iron matrix. Here, when the hardness of the second hard particles is less than 600 Hv, the wear resistance of the sintered alloy may decrease.

The hardnesses of the first and second hard particles can be adjusted by appropriately setting proportions of the components in the above content range, the content of the graphite particles, the sintering temperature and the sintering time. The sintering temperature may be about 1050° C. to 1250° C., and particularly, about 1100° C. to 1150° C. The sintering time at the above sintering temperature may be 30 minutes to 120 minutes, and more preferably 45 minutes to 90 minutes. The sintering atmosphere may be a non-oxidizing atmosphere such as an inert gas atmosphere. As the a non-oxidizing atmosphere, a nitrogen gas atmosphere, an argon gas atmosphere, a vacuum atmosphere, and the like may be used.

The matrix of the iron-based sintered alloy obtained by sintering preferably includes a structure containing pearlite in order to secure its hardness. The structure containing pearlite may be a pearlite structure, a mixed pearlite-austenitic structure, or a mixed pearlite-ferrite structure. In order to secure wear resistance, ferrite with low hardness is preferably contained in a small amount.

After the sintered product is prepared, an oxidation treatment is performed on the sintered product so that a part of iron contained in the iron matrix derived from the iron particles become triiron tetraoxide (Fe₃O₄). The oxidation treatment is performed so that a density difference between before and after the oxidation treatment in the sintered product becomes 0.05 g/cm³ or more. In the oxidation treatment, oxides mainly including triiron tetraoxide are generated. Therefore, the mass of the sintered product after the oxidation treatment increases. Therefore, a higher density difference indicates a larger amount of triiron tetraoxide generated.

When a density difference between before and after the oxidation treatment in the sintered product is set to 0.05 g/cm³ or more, it is possible to improve the wear resistance of the sintered alloy. Here, when a density difference between before and after the oxidation treatment in the sintered product is less than 0.05 g/cm³, since a proportion of triiron tetraoxide in the sintered alloy is small, adhesive wear is accelerated due to metal contact with a counterpart member. As a result, the wear resistance of the sintered alloy decreases.

In such an oxidation treatment, for example, under a water vapor atmosphere, the sintered product is heated in temperature conditions of 500° C. to 600° C. for 30 minutes to 90 minutes. Therefore, in the above range of the density difference, iron (Fe) which is a matrix of the sintered product can be oxidized to triiron tetraoxide (Fe₃O₄).

7. Application of Wear-Resistant Iron-Based Sintered Alloy

The sintered alloy obtained in the above production method has a higher mechanical strength and wear resistance under a high temperature usage environment than those in the related art. For example, it can be suitably used for a valve system (for example, a valve seat and a valve guide) and a wastegate valve of a turbocharger of an internal combustion engine in which compressed natural gas or liquefied petroleum gas is used as a fuel under a high temperature usage environment.

For example, when a valve seat of an exhaust valve of an internal combustion engine is made of a sintered alloy, even if a wear pattern in which adhesive wear when the valve seat and the valve come in contact with each other and abrasive wear when the two slide over each other are combined develops, the wear resistance of such a valve seat is improves compared with in the related art. In particular, under a usage environment in which compressed natural gas or liquefied petroleum gas is used as a fuel, although a Mo oxide film is unlikely to be formed, in such an environment, it is possible to reduce adhesive wear.

Examples in which the present disclosure is realized in practice will be described below together with comparative examples.

EXAMPLE 1 Optimal Amount of First Hard Particles Added

A sintered alloy according to Example 1 was produced according to the following production method. As the first hard particles, hard particles (commercially available from Daido Steel Co., Ltd) produced from an alloy including Mo: 40 mass %, Ni: 30 mass %, Co: 20 mass %, Mn: 5 mass %, Si: 0.8 mass %, and C: 1.2 mass %, with the balance including Fe and inevitable impurities (that is, Fe-40Mo-30Ni-20Co-5Mn-0.8Si-1.2C) using a gas atomizing method were prepared. The first hard particles were classified into a range of 44 μm to 250 μm using a sieve according to JIS standard Z8801. Here, “granularity of particles” in this specification is a value obtained by classification according to this method.

As the second hard particles, second hard particles (commercially available from Kinsay Matec Co., Ltd) produced from an Fe-65 alloy including Mo: 65 mass %, with the balance including Fe and inevitable impurities using a grinding method were prepared. The second hard particles were classified into a range of 75 μm or less.

Next, a graphite powder (CPB-S commercially available from Nippon Kokuen Group) including graphite particles and a reduced iron powder (JIP255M-90 commercially available from JFE Steel Corporation) including pure iron particles were prepared. The above first hard particles, second hard particles, and graphite particles in proportions of 40 mass %, 3 mass %, and 1.1 mass %, respectively, with the remaining iron particles (specifically 55.9 mass %) were mixed together using a V type mixer for 30 minutes. Thereby, a mixed powder was obtained.

Next, using a molding die, the obtained mixed powder was compact-molded into a ring-shaped test piece at a pressurizing force of 588 MPa to form a molded product for a sintered alloy (compact-molded product). The compact-molded product was sintered in an inert atmosphere (nitrogen gas atmosphere) at 1120° C. for 60 minutes to obtain a sintered product. The sintered product was oxidized by heating under a water vapor atmosphere in heating conditions of 550° C. for 50 minutes. Thereby, a sintered alloy (valve seat) test piece according to Example 1 was formed.

EXAMPLES 2 and 3 Optimal Amount of First Hard Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Examples 2 and 3 were examples for evaluating an optimal amount of first hard particles added. Examples 2 and 3 differed from Example 1 in that, as shown in Table 1, the first hard particles were added at a proportion of 5 mass % and 50 mass %, respectively, with respect to the entire mixed powder.

EXAMPLES 4 and 5 Optimal Amount of Second Hard Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Examples 4 and 5 were examples for evaluating an optimal amount of second hard particles added. Examples 4 and 5 differed from Example 1 in that, as shown in Table 1, the second hard particles were added at a proportion of 1 mass % and 5 mass %, respectively, with respect to the entire mixed powder.

EXAMPLES 6 and 7 Optimal Amount of Graphite Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Examples 6 and 7 were examples for evaluating an optimal amount of graphite particles added. Examples 6 and 7 differed from Example 2 in that, as shown in Table 1, the graphite particles were added at a proportion of 0.5 mass % and 1.5 mass %, respectively, with respect to the entire mixed powder.

EXAMPLE 8 Hardness of First Hard Particles

In the same manner as in Example 1, a sintered alloy test piece was prepared. Example 8 differed from Example 1 in that the sintering temperature was lower than in Example 1 and the hardnesses of the first hard particles of the sintered product after sintering was lowered (refer to Table 1, 545 Hv).

Comparative Examples 1 and 2 Comparative Examples for Optimal Amount of First Hard Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Comparative Examples 1 and 2 were comparative examples for evaluating an optimal amount of first hard particles added. Comparative Examples 1 and 2 differed from Example 1 in that, as shown in Table 1, the first hard particles were added at a proportion of 0 mass % (that is, not added) and 60 mass %, respectively, with respect to the entire mixed powder. Here, in Comparative Example 2, it was not possible to mold a molded product from the mixed powder.

Comparative Examples 3 and 4 Comparative Examples for Optimal Amount of Second Hard Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Comparative Examples 3 and 4 were comparative examples for evaluating an optimal amount of second hard particles added. Comparative Examples 3 and 4 differed from Example 1 in that, as shown in Table 1, the second hard particles were added at a proportion of 0 mass % and 10 mass %, respectively, with respect to the entire mixed powder. In addition, in Comparative Example 3, the graphite particles were added at a proportion of 0.8 mass %.

Comparative Examples 5 and 6 Comparative Examples for Optimal Amount of Graphite Particles Added

In the same manner as in Example 1, sintered alloy test pieces were prepared. Comparative Examples 5 and 6 were comparative examples for evaluating an optimal amount of graphite particles added. Comparative Examples 5 and 6 differed from

Example 1 in that, as shown in Table 1, the graphite particles were added at a proportion of 0.4 mass % and 1.6 mass %, respectively, with respect to the entire mixed powder.

Comparative Example 7 comparative Example for Density Difference in Sintered Product

In the same manner as in Example 1, a sintered alloy test piece was prepared. In Comparative Example 7, a molding pressure during compact-molding was higher than in Example 1, and the density before the oxidation treatment was higher. Therefore, there were less pores inside the sintered product, and thus generation of oxides was prevented and an increase in density of the sintered product after the oxidation treatment was reduced (that is, the density difference was reduced).

Comparative Example 8 Comparative Example for Hardness of First Hard Particles

In the same manner as in Example 1, a sintered alloy test pieces was prepared. Comparative Example 8 differed from Example 1 in that the sintering temperature was higher than in Example 1 and the hardness of the first hard particles of the sintered product after sintering was higher (refer to Table 1, 650 Hv).

Comparative Example 9

In the same manner as in Example 1, a sintered alloy test piece was prepared. Comparative Example 9 differed from Example 1 in that particles including a Co-40Mo-5Cr-0.9C alloy corresponding to the hard particles described in JP 2004-156101 A were used as the first hard particles, no second hard particles were added, and no oxidation treatment were performed on the sintered product after sintering.

<Hardness Test>For the sintered alloy test pieces according to Examples 1 to 8 and Comparative

Examples 1 to 9, the hardnesses of the first hard particles and the second hard particles were measured using a micro Vickers hardness testing machine at a measurement load of 0.1 kgf. The results are shown in Table 1.

<Density Measurement Test>

Masses of the sintered alloy test pieces according to Examples 1 to 8 and Comparative Examples 1, and 3 to 8 before and after the oxidation treatment were measured. The measured mass was divided by a volume calculated from the size of the test piece, and densities of the test piece (sintered product) before and after the oxidation treatment were calculated. In addition, a density difference between before and after the oxidation treatment in the test piece (sintered product) was calculated. The results are shown in Table 1.

<Wear Test>

Using a testing machine in FIG. 1, a wear test was performed on the sintered alloy test pieces according to Examples 1 to 8 and Comparative Examples 1, and 3 to 9, and wear resistances thereof were evaluated. In this test, as shown in FIG. 1, a propane gas burner 10 was used as a heating source, and a sliding part between a ring-shaped valve seat 12 made of the sintered alloy prepared as above and a valve face 14 of a valve 13 was placed in a propane gas combustion atmosphere. The valve face 14 was obtained by performing nitrocarburizing according to EV12 (SEA standard). The temperature of the valve seat 12 was controlled such that it was 250° C., a load of 25 kgf was applied by a spring 16 when the valve seat 12 was brought into contact with the valve face 14, the valve seat 12 was brought into contact with the valve face 14 at 3250 times/min, and the wear test was performed for 8 hours.

The total amount of a wear depth in the axial direction of the valve seat 12 and the valve face 14 after the wear test was measured as a wear test wear amount, and a value obtained by dividing the wear test wear amount by the value in Comparative Example 9 was calculated as a wear test wear amount proportion. The results are shown in Table 1.

FIGS. 3A, 4A, 5A, 6A, and 7A show plotted results of corresponding wear test wear amount proportions among Examples 1 to 8 and Comparative Examples 1, and 3 to 9 in which the horizontal axis represents the amount of first hard particles added, the amount of second hard particles added, the amount of graphite particles added, the hardness of the first hard particles, and the density difference in the sintered product in that order.

In addition, surfaces of the test pieces after the wear test according to Example 1 and Comparative Example 7 after the wear test were observed under a microscope. The results are shown in FIG. 8A and FIG. 8B. FIG. 8A is a picture of the surface of the test piece according to Example 1 after the wear test and FIG. 8B is a picture of the surface of the test piece according to Comparative Example 7 after the wear test.

The test pieces of Example 1, Comparative Example 5, and Comparative Example 6 before the wear test were etched using nital, and the structure of the sintered alloy was observed under a microscope. The results are shown in FIG. 9A to FIG. 9C. FIG. 9A is a picture of the structure of the test piece according to Example 1, FIG. 9B is a picture of the structure of the test piece according to Comparative Example 5, and FIG. 9C is a picture of the structure of the test piece according to Comparative Example 6.

<Machinability Test>

Using a testing machine shown in FIG. 2, a machinability test was performed on the sintered alloy test pieces according to Examples 1 to 8 and Comparative Examples 1, and 3 to 9, and the machinability thereof was evaluated. In this test, six test pieces 20 with an outer diameter of 30 mm, an inner diameter of 22 mm, and a total length of 9 mm were prepared for each of Examples 1 to 8 and Comparative Examples 1, and 3 to 9. Using an NC lathe, a test piece 20 rotated at a rotational speed of 970 rpm was cut in a wet type traverse using a titanium nitride aluminum-coated cemented carbide tool (cutting tool) 30 with a cut depth of 0.3 mm, while being fed at 0.08 mm/rev, and over a cutting distance of 320 m. Then, a maximum wear depth of a flank of the tool 30 was measured as a tool wear amount using an optical microscope and a value obtained by dividing the tool wear amount by the value in Comparative Example 9 was calculated as a tool wear amount proportion. The results are shown in Table 1.

FIGS. 3B, 4B, 5B, 6B, and 7B show plotted results of corresponding tool wear amount proportions among Examples 1, and 3 to 8 and Comparative Examples 1 to 9 in which the horizontal axis represents the amount of first hard particles added, the amount of second hard particles added, the amount of graphite particles added, the hardness of the first hard particles, and the density difference in the sintered product in that order.

TABLE 1 First hard Second hard Oxidation treatment Graphite particles particles Before After particles Wear test Amount Amount treatment treatment Density Amount wear Tool wear Sample Hardness added Hardness added Density Density difference added amount amount name (Hv) (mass %) (Hv) (mass %) (g/cm³) (g/cm³) (g/cm³) (mass %) proportion proportion Example 1 560 40 1080 3 6.96 7.09 0.13 1.1 0.12 0.69 Example 2 568 5 940 3 7.07 7.13 0.06 1.1 0.33 0.59 Example 3 555 50 920 3 6.89 7.10 0.21 1.1 0.10 0.77 Example 4 570 40 1000 1 7.00 7.15 0.15 1.1 0.14 0.56 Example 5 585 40 1090 5 6.92 7.10 0.18 1.1 0.13 0.86 Example 6 566 40 1020 3 6.98 7.13 0.15 0.5 0.18 0.66 Example 7 562 40 960 3 7.00 7.08 0.08 1.5 0.13 0.84 Example 8 545 40 980 3 6.80 6.99 0.19 1.1 0.06 0.67 Comparative — 0 1010 3 7.03 7.10 0.07 1.1 0.46 0.54 Example 1 Comparative 570 60 990 3 — — — 1.1 Not Not Example 2 possible possible to mold to mold Comparative 560 40 — 0 6.98 7.12 0.14 0.8 0.20 0.54 Example 3 Comparative 577 40 1080 10 6.76 7.00 0.24 1.1 0.10 0.97 Example 4 Comparative 560 40 1000 3 6.97 7.12 0.15 0.4 0.34 0.65 Example 5 Comparative 560 40 990 3 6.78 7.04 0.26 1.6 0.13 0.94 Example 6 Comparative 540 40 1100 3 7.16 7.20 0.04 1.1 0.42 0.70 Example 7 Comparative 650 40 990 3 6.96 7.12 0.16 1.1 0.13 0.90 Example 8 Comparative 880 40 None — — — — 1.1 1.00 1.00 Example 9

(Result 1: Optimal Amount of First Hard Particles Added)

As shown in FIG. 3A, the wear test wear amount proportions of Examples 1 to 3 were lower than in Comparative Examples 1 and 9. The wear test wear amount proportions decreased in the order of Example 2, Example 1, and Example 3. Therefore, when the first hard particles were added, the abrasive wear resistance of the sintered alloy was assumed to be improved. However, in Comparative Example 2, it can be said that moldability of the molded product deteriorated because too much first hard particles were added. Based on the above, the optimal amount of first hard particles added was 5 mass % to 50 mass % with respect to the entire mixed powder.

Here, as shown in FIG. 3B, the tool wear amount proportions of Examples 1 to 3 were smaller than in Comparative Example 9. The tool wear amount proportion increased in the order of Example 2, Example 1, and Example 3. However, it is thought that, when more first hard particles were added than in Example 3, the machinability of the sintered alloy deteriorated and the tool wear amount proportion increased.

(Result 2: Optimal Amount of Second Hard Particles Added)

As shown in FIG. 4A, the wear test wear amount proportions of Examples 1, 4, and 5 and Comparative Example 4 were lower than in Comparative Examples 3 and 9. However, as shown in FIG. 4B, the tool wear amount proportion of Comparative Example 4 was higher than in Examples 1, 4, and 5. Here, when the surface of the test piece after the wear test was observed, Comparative Example 3 had more scratches due to adhesive wear than the other examples.

Therefore, it is thought that the second hard particles improved the hardness of the sintered alloy after sintering, prevented plastic deformation of the iron matrix of the sintered alloy during use, and reduced adhesive wear of the sintered alloy. Specifically, it is thought that, since the second hard particles did not contain Ni, Co, and the like unlike the first hard particles, the iron matrix around the second hard particles was able to harden than in the first hard particles, molybdenum carbide precipitated at the grain boundary of the iron matrix during sintering, and thus the hardness of the iron matrix after sintering was improved.

Based on the above, when an amount of second hard particles added was too small, the surface of the sintered alloy after the wear test was easily scraped off. On the other hand, it is thought that, as in Comparative Example 4, when an amount of second hard particles added was too large, the sintered alloy after sintering was too hard, and machinability deteriorated. Based on the above result, the optimal amount of second hard particles added was 1 mass % to 5 mass % with respect to the entire mixed powder.

(Result 3: Optimal Amount of Graphite Particles Added)

As shown in FIG. 5A, the wear test wear amount proportions of Examples 1, 6, and 7 and Comparative Example 6 were lower than in Comparative Examples 5 and 9. However, as shown in FIG. 5B, the tool wear amount proportion of Comparative Example 6 was higher than in Examples 1, 6, and 7.

As shown in FIG. 9A, in the structure of the sintered alloy shown in Example 1, a pearlite structure was formed. However, as shown in FIG. 9C, in the structure of the sintered alloy shown in Comparative Example 6, a cementite structure was formed due to an increased amount of graphite particles. It is thought that, as a result, the tool wear amount proportion of Comparative Example 6 was higher than in Examples 1, 6, and 7.

On the other hand, it is thought that, as shown in FIG. 9B, in the structure of the sintered alloy shown in Comparative Example 5, since the structure had ferrite as a major part thereof, the wear test wear amount proportion of Comparative Example 5 was higher than in Examples 1, 6, and 7 and Comparative Example 6. Therefore, the optimal amount of graphite particles added at which it was possible to secure a pearlite structure in the iron matrix after sintering was 0.5 mass % to 1.5 mass % with respect to the entire mixed powder.

(Result 4: Optimal Hardness of First Hard Particles)

As shown in FIG. 6A, the wear test wear amount proportions of Examples 1, 3, 5, and 8 and Comparative Example 8 were lower than in Comparative Example 9. However, as shown in FIG. 6B, the tool wear amount proportion of Comparative Example 8 was higher than in Examples 1, 3, 5, and 8.

It is thought that, since the hardness of the first hard particles in Comparative Example 9 was higher than in Examples 1, 3, 5, and 8 and Comparative Example 8, a counterpart component wore more, and the wear test wear amount proportion of Example 9 was higher than in the other examples. On the other hand, it is thought that, in Examples 1, 3, 5, and 8, since the hardness (Hv) of the first hard particles was lower than in Comparative Example 8 and was 600 or less, the tool wear amount proportions of Examples 1, 3, 5, and 8 were lower than in Comparative Example 8. Here, in Examples 1, 3, 5, and 8, it can be said that, since first hard particles with a hardness of 400 Hv or more were secured, wear resistance was secured.

Therefore, the hardness of the first hard particles after sintering is preferably in a range of 400 to 600 Hv. Here, in consideration of the second hard particles that improved the wear resistance of the iron matrix, in the condition of the above range of the amount added, it was necessary for the hardness of the second hard particles to be higher than the hardness of the first hard particles and to exceed at least 600 Hv.

(Result 5: Optimal Density Difference in Sintered Product)

As shown in FIG. 7A, the wear test wear amount proportions of Examples 1 to 8 were lower than in Comparative Example 7 and 9. As shown in FIG. 7B, the tool wear amount proportion of Comparative Example 9 was higher than in Examples 1 to 8, and Comparative Example 7.

In Comparative Example 7, since a density difference between before and after the oxidation treatment in the sintered product was less than 0.05 g/cm³, an amount of oxides mainly including triiron tetraoxide in the sintered product was smaller than in the sintered products of Examples 1 to 8. Therefore, metal contact with a counterpart component was promoted, and as shown in FIG. 8B, in the test piece (sintered product) of Comparative Example 7, and it is thought that adhesive wear with a counterpart component accelerated. On the other hand, in Examples 1 to 8, the wear resistance of the sintered alloy is thought to have been higher than in Comparative Example 7 since there was almost no such adhesive wear, (for example, refer to Example 1, FIG. 8A). Therefore, it was necessary to perform an oxidation treatment so that a density difference between before and after the oxidation treatment in the sintered product became 0.05 g/cm³ or more.

EXAMPLE 9 Optimal Particle Size of Second Hard Particles

In the same manner as in Example 1, a sintered alloy test piece was prepared. Example 9 was an example for evaluating an optimal particle size of the second hard particles. Example 9 differed from Example 1 in that, as the second hard particles, second hard particles classified to have a particle size (granularity) in a range of greater than 75 μm and 100 μm or less were used.

Comparative Example 10 Comparative Example for Optimal Particle Size of Second Hard Particles

In the same manner as in Example 1, a sintered alloy test piece was prepared. Comparative Example 10 is a comparative example for evaluating an optimal particle size of the second hard particles. Comparative Example 10 differed from Example 1 in that, as the second hard particles, second hard particles classified to have a particle size in a range of greater than 100 μm and 150 μm or less were used. Here, the test piece according to Comparative Example 10 was a sintered alloy included in the scope of the present disclosure and was set as Comparative Example 10 for convenience to allow comparison with Examples 1 and 9.

In the same manner as in Example 1, the wear test and the machinability test were performed on the test pieces of Example 9 and Comparative Example 10, and a wear test wear amount and a tool wear amount were measured. The results are shown in FIG. 10A and FIG. 10B together with the above results of Example 1.

FIG. 10A is a graph showing results of the wear test wear amount proportions in Examples 1 and 9 and Comparative Example 10 and FIG. 10B is a graph showing results of the tool wear amount proportions in Examples 1 and 9 and Comparative Example 10.

(Result 6: Optimal Particle Size of Second Hard Particles)

As shown in FIG. 10A, the wear test wear amount proportions of Examples 1 and 9 and Comparative Example 10 were similar. However, as shown in FIG. 10B, the tool wear amount proportions of Examples 1 and 9 were lower than in Comparative Example 10, and the tool wear amount proportion of Example 1 was the lowest among the examples. This is because, in Comparative Example 10, since the particle size of the second hard particles was too large, the machinability of the test piece (sintered product) deteriorated in some cases. Based on this result, the particle size (maximum particle size) of the second hard particles was preferably in a range of 100 μm or less, and more preferably, the particle size (maximum particle size) of the second hard particles was in a range of 75 μm or less.

While embodiments of the present disclosure have been described above in detail, the present disclosure is not limited to these embodiments, and various design modifications can be made. 

What is claimed is:
 1. A method of producing a wear-resistant iron-based sintered alloy comprising: a molding process in which a mixed powder including hard particles, graphite particles, and iron particles is compact-molded into a molded product for a sintered alloy; and a sintering process in which the molded product for the sintered alloy is sintered while C of the graphite particles of the molded product for the sintered alloy diffuses into the hard particles and the iron particles, wherein the hard particles include first hard particles and second hard particles, wherein the first hard particles include Mo: 20 mass % to 70 mass %, Ni: 5 mass % to 40 mass %, Co: 5 mass % to 40 mass %, Mn: 1 mass % to 20 mass %, Si: 0.5 mass % to 4.0 mass %, and C: 0.5 mass % to 3.0 mass %, with the balance including Fe and inevitable impurities when an amount of the first hard particles is set as 100 mass %, wherein the second hard particles include Mo: 60 mass % to 70 mass %, and Si: 2.0 mass % or less, with the balance including Fe and inevitable impurities when an amount of the second hard particles is set as 100 mass %, wherein the mixed powder includes the first hard particles at 5 mass % to 50 mass %, the second hard particles at 1 mass % to 5 mass %, and the graphite particles at 0.5 mass % to 1.5 mass % when a total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set as 100 mass %, and wherein, in the sintering process, sintering is performed so that the hardness of the first hard particles becomes 400 to 600 Hv and the hardness of the second hard particles exceeds 600 Hv, after the sintering process, an oxidation treatment is performed on a sintered product sintered from the molded product for the sintered alloy so that a part of iron contained in an iron matrix derived from the iron particles becomes triiron tetraoxide, and the oxidation treatment is performed so that a difference between a density of the sintered product before the oxidation treatment and a density of the sintered product after the oxidation treatment becomes 0.05 g/cm³ or more.
 2. The method according to claim 1, wherein 10 mass % or less of Cr is additionally added to the first hard particles when the amount of the first hard particles is set as 100 mass %.
 3. The method according to claim 1, wherein a particle size of the second hard particles is in a range of 100 μm or less. 